Problem: Integrate. $ \int 5\sec^2(x)\,dx $ Choose 1 answer: Choose 1 answer: (Choice A) A $5\tan(x)+C$ (Choice B) B $5\csc(x)+C$ (Choice C) C $5\sec(x)+C$ (Choice D) D $5\cot(x)+C$
Solution: We need a function whose derivative is $5\sec^2(x)$. We know that the derivative of $\tan(x)$ is $\sec^2(x)$, so let's start there: $\dfrac{d}{dx} \tan(x) = \sec^2(x)$ Now let's multiply by $5$ : $\dfrac{d}{dx}\left[ 5\tan(x) \right] = 5\dfrac{d}{dx} \tan(x) = 5\sec^2(x)$ Because finding the integral is the opposite of taking the derivative, this means that: $ \int 5\sec^2(x)\,dx =5 \tan(x)\, + C$ The answer: $5 \tan(x)\, + C$